1. Field of the Invention
The present invention relates to a semiconductor device. More particularly, the present invention relates to a multi-level non-volatile memory, a manufacturing method thereof, and an operation method thereof.
2. Description of Related Art
Non-volatile memory device has been widely applied to personal computers and electronic apparatuses for the operations of data writing, reading, and erasing can be performed repeatedly to a non-volatile memory device and the data stored in a non-volatile memory device will not be lost even after power supply is turned off.
The floating gate and control gate of a typical non-volatile memory device are fabricated with doped polysilicon. The control gate is disposed on the floating gate directly, the floating gate and the control gate are isolated by a dielectric layer, and the floating gate and the substrate are isolated by a tunneling oxide (the so-called stacked gate flash memory). Moreover, to resolve the problem of data misjudgment due to over-erase when erasing the memory, a select gate is further disposed on the sidewalls of the control gate and the floating gate, above the substrate to form a split gate structure.
Generally speaking, the commonly-used non-volatile memory device is of, for example, NOR array structure. In a conventional NOR array, every two memory cells share the contact window of a bit line and a source line. Thus, each memory cell takes up the size of half contact window and half source line. In the NOR array, reading and writing operations can be performed randomly to the memory cells since every memory cell is directly connected by a bit line, and the current of a memory cell during reading operation is larger, accordingly the reading speed is faster, since the resistor connected in series is smaller.
However, when a memory in the NOR non-volatile memory device is used as a multi-level memory cell, the threshold voltage for determining various data states has small distribution range, thus, in the programming operation of conventional NOR non-volatile memory, multiple programming steps and programming confirmation steps have to be executed to make the threshold voltage of the programmed memory cell to be within a predetermined distribution range, which may require a long time.